Over the last 100 years, the primary causes of morbidity and mortality in the United States have changed from infectious diseases to chronic diseases. The public health burden of these chronic diseases can be reduced by changing health behaviors such as physical activity and nutrition. However, surprisingly little is known about the mechanisms of change for physical activity and nutrition that can inform the design of successful intervention. Therefore, the primary aim of this project is to identify the mechanisms of behavior change for physical activity (PA) and nutrition (fruit and vegetable consumption) using a novel application of one of the most powerful frameworks of health behavior change - the Transtheoretical Model of Behavior Change (TTM). Specifically: (1) We will determine whether the stages of change, processes, pros, cons, self-efficacy and temptations defined by the TTM moderate and mediate, respectively, self-initiated health behavior changes over a 2 year period among a random, multi-ethnic sample. Due to the longitudinal and multi-behavioral nature of this project we will also be able to investigate the following secondary aims: 1) To identify the longitudinal relationship between behavior change and stage change for PA and fruit and vegetable consumption. 2) To evaluate the impact that the PA mechanisms have on fruit and vegetable consumption and vice versa. The aims will be addressed via a naturalistic longitudinal design assessing all TTM constructs (stage, processes, pros, cons, self-efficacy [confidence and temptations]) at baseline and 3, 6, 9, 12, 18, and 24 months follow-up. This type of rigorous comprehensive approach has not been conducted using the entire TTM for PA and nutrition behaviors and directly addresses several core model assumptions and criticisms. Further, including two behaviors will allow us to look at the gateway behavior issue - does change in one health behavior area lead to change in another, which has important practice implications.